Internal combustion engines customarily include bores containing reciprocal pistons that compress a flammable mixture of air and fuel for combustion. The delivery of air and, sometimes fuel, into the engine cylinders, and also the evacuation of exhaust gas produced as a byproduct of oxidation of the flammable mixture within the cylinder, is generally controlled by poppet valves and fuel injectors, the operation of which can be mechanical and/or electrical or hydraulic. For mechanical valve systems, poppet valves are typically used, which are activated in a reciprocal fashion by a camshaft having a follower acted upon. In some instances, a unit fuel injector may also be used to pressurize and inject a predefined quantity of fuel into the cylinder. The pressurization of the fuel is accomplished by a plunger, which can also be activated mechanically by a follower or lifter that is in contact with a rotating camshaft. A mechanical fuel delivery arrangement is especially advantageous for certain severe service applications such as for marine or locomotive engine applications.
For engines that include mechanical unit injection fuel systems, in addition to mechanical valve activation systems, a minimum of three camshaft lobes is required for each engine cylinder. As is known, a lobe is an eccentric feature of a camshaft that converts the rotational motion of the camshaft into a reciprocal axial motion of a camshaft follower, which is used to activate other engine components. Therefore, for an engine having a mechanical unit injection system, one lobe can be used to activate the fuel injection, and the remaining two lobes can be used to activate the intake and exhaust valves, respectively. For engines having multiple intake and/or exhaust valves per cylinder, or more than one fuel injection type, additional lobes may be used per engine cylinder.
As can be appreciated, multiple lobes corresponding to each engine cylinder can create packaging space constraints. The challenge with spacing is exacerbated for engines having cylinders in opposing relation such as vee-engines, which will typically include one camshaft per engine cylinder bank, which is placed on the outboard or inboard side of the engine. In these known arrangements, however, outboard camshafts result in a more complex geartrain arrangement to drive the camshafts and increase overall engine width. Likewise, previously proposed inboard camshaft arrangements can increase the driving geartrain complexity and also reduce the torsional rigidity of the driving mechanism, which over time can lead to inefficient engine operation and increased wear on the various engine components associated with the power cylinders.
One example of a previously proposed engine configuration in which two camshafts are placed in the valley of a vee-engine cylinder case can be seen in U.S. Pat. No. 5,564,395 to Moser et al. (“Moser”). Moser describes an engine having a V-shaped block in which two camshafts are placed. A first camshaft operates pushrods connected to rocker arms that activate the engine's intake and exhaust valves, and a second camshaft operates roller elements associated with pump elements, which are also placed within the valley of the V-shaped engine block. For driving the two camshafts, the engine described in Moser includes a first gear drive that establishes a direct connection between a crankshaft of the engine and the first camshaft driving the intake and exhaust valves, and a second gear drive that establishes a direct connection between the first camshaft and the second camshaft driving the pumping elements. While the dual camshaft arrangement of camshafts described in Moser is at least partially effective in alleviating space constraints, the indirect driving of the second camshaft by the first camshaft can increase the torsional elasticity of the valve and pumping unit drive system of the engine, which can also increase engine-to-engine performance variability and component wear over time.